A Very Low Luminosity, Very Cool, DC White Dwarf

Abstract

The star LHS 3250 is found to be a white dwarf at a distance of 30 pc. Its absolute magnitudes (M V = 15.72; M bol = 16.2) put it among the least-luminous white dwarfs known. Its optical spectrum shows no features, indicating it has a DC classification, and it shows no detectable polarization, indicating it does not have a very strong magnetic field. However, its broadband colors show it to have a unique spectral energy distribution, and it stands out from all other stars in BV I and other broadband photometric surveys.We discuss these properties, and conclude that LHS 3250 must be an extremely cool white dwarf with strong collision-induced absorption at red-infrared wavelengths from molecular hydrogen, in accord with models for very cool white dwarf atmospheres. If so, it is the first such star known, and the first star to provide observational evidence supporting these models. It suggests that other very cool white dwarfs, both halo white dwarfs and the oldest disk white dwarfs, also may have colors affected by similar absorption. The atmospheric composition of LHS 3250 is not known, and therefore its temperature is poorly determined. It may be a helium-core star with a mass 0.3 < M < 0.45M ⊙ and a product of mass-transfer in a close binary system. However, until its temperature is better known, its mass and age remain uncertain.

The Halo White Dwarf WD 0346+246 Revisited

“…5). It thus seems extremely dangerous to interpret these newly discovered ultracool white dwarfs in two-color or color magnitude diagrams that involve "" blue ÏÏ magnitudes (see, e.g., Harris et al 1999Harris et al , 2001Oppenheimer et al 2001aOppenheimer et al , 2001b) until this problem with the missing opacity has been properly dealt with and included in the theoretical models and colors.…”

“…Cool white dwarfs with mixed compositions thus easily stand out in such diagrams. LHS 1126 represents the Ðrst white dwarf successfully interpreted in terms of CIAs in a mixed hydrogen/helium H 2 -He atmosphere (Bergeron et al 1994) ; it is important to point out in this context that the case of LHS 1126 is a deÐnite conÐrmation of the presence of CIAs in a cool white H 2 -He dwarf and not a possible identiÐcation, as suggested by Harris et al (1999).…”

“…For instance, the energy distributions of LHS 3250 and its almost identical twin SDSS 1337]00 show a maximum peak that is considerably broader and bluer than predicted from pure hydrogen model atmospheres (Harris et al 1999(Harris et al , 2001. The discrepancy between the models and the observations has been attributed to some missing opacity, most likely originating from absorptions by molecular hydrogen due to collisions with neutral helium, although this interpretation could not be conÐrmed quantitatively (Harris et al 1999 ;Oppenheimer et al 2001b). The temperature and composition of these two white dwarfs thus remain unknown.…”

Infrared Absorption Spectra of Collisionally Interacting He and H Atoms

“…Collision-induced absorption of compressed, nonpolar gases and gas mixtures, such as hydrogen with or without helium admixtures, were discovered in 1949 (Welsh, Crawford, & Locke 1949), and their signiÐcance for astrophysics was almost instantly recognized (Welsh 1972 ;Herzberg 1952aHerzberg , 1952bTrafton 1964 Borysow, & Orton 1996) which are based on advanced ab initio quantum chemical calculation ; these models were shown to reproduce virtually all existing laboratory measurements of the kind with great accuracy. These models together with computed quantum line shapes have been found useful in the various analyses of the infrared spectra of the outer planets and, more recently, for analyses of the atmospheres of certain cold stars (Borysow, & JÔrgensen, Zheng 1997 ;Burrows et al 1997 ;Hansen 1998Hansen , 1999Harris et al 1999 ;Hodgkin et al 2000).…”

Cosmological parameters

“…The key observation is that there is a lower limit to the luminosity, and therefore also the temperature, of nearby white dwarfs; although dimmer ones could have been seen, none have been found (cf. however [53]). The only plausible explanation is that the white dwarfs have not had sufficient time to cool to lower temperatures, which initially led to an estimate of 9.3 ± 2 Gyr for the age of the Galactic disk [130].…”